1. Field of the Invention
This invention relates to the field of miniature electrical component measuring and testing. More particularly, it relates to a device to enhance the performance of high resistance electrical measuring instruments, such as the Hewlett Packard 4349B, and to a method of using that device to increase the sensitivity of such instruments in the xe2x80x9cParts Presentxe2x80x9d field below the minimum limits of their existing ranges.
2. Description of the Prior Art
In the field of miniature ceramic electrical capacitor chips, that may measure as small as 0.020xc3x970.020xc3x970.040 inches in external dimensions and exhibit a capacitance below one picofarad, it is well-known there is present a finite parallel conductance that results in some DC leakage current through the part. As capacitor chips are essential to the operation of computer and many other electronic instruments, it is essential to measure this DC (direct current) leakage current and to classify these chips accordingly. In the prior art, high resistance measuring instruments such as the Hewlett Packard Model 4349B are used.
In the ceramic capacitor (chip) industry, Hewlett Packard 4349B meters have been coupled to high speed component xe2x80x9chandlersxe2x80x9d or processing machines to measure the DC leakage current through these capacitors at high throughput rates. The handlers receive untested capacitor chips, which, either individually or in multiples, bring them to test stations at which the handlers connect the capacitor terminals to test instruments, (such as a current limited power supply on one side and the HP 4349B or other such high resistance/low current measuring device on the other), for DC leakage testing. Prior to or after leakage testing, the measuring instrument conducts a test protocol, xe2x80x9cPart Presentxe2x80x9d or xe2x80x9cContact Checkxe2x80x9d, to verify the presence of a capacitor at the test station which is satisfactorily connected on both sides to the test circuit. The reason for this protocol is to assure that the part at the test station is satisfactorily connected into the test circuit, because otherwise a part with high leakage characteristics could be classified as a xe2x80x9cgoodxe2x80x9d part when in fact it is not!
In the case of the BP 4349B, a triax cable connects the instrument to the LO side of the D.U.T. (device Under Test); the inner conductor of this cable carries the DC current signal from the D.U.T.; the inner shield carries an HF (high frequency) guard signal of approximately 0.5 MHz (Mega Hertz) generated within the 4349B at the time of the xe2x80x9cParts Presentxe2x80x9d test and the outer shield of the triax is grounded.
Now, the HI side of the D.U.T., which connects directly to the power supply, is, at high frequencies, essentially grounded. During the xe2x80x9cParts Presentxe2x80x9d test the HF signal that drives the guard is applied by the 4349B at the same amplitude to the inner conductor of the triax cable. In the absence of a satisfactory connection to a present or absent ceramic capacitor, the HF current flowing through the guard (the outer end of which is open) will therefore equal that flowing through the inner conductor and no part will be detected. In the event of a satisfactory connection at the test station to a present part, however, these currents will be unequal and a part present with satisfactory connection will be recognized by the 4349B. In the event that a xe2x80x9cNO PART PRESENTxe2x80x9d classification is made, this classification demands a retest of any part that may have been present in the test station and takes priority over any classification that is derived from the leakage current test.
The failing of this prior art practice is that parts with a capacitance value of less than 2pF often go undetected, even though a satisfactory contact is made thereto at the test station.
This invention is a device and method of using that device which permits the HP4349B to determine whether a part is present at the test station and a good contact made thereto, even when that part has a capacitance value as low as 0.75 pF. With respect to the device, it comprises a xe2x80x9csignal boosterxe2x80x9d which may be a transformer, amplifier or other equivalent component with a differential input driven from the inner guard shield and outer ground shield of the HP4349B input triax cable. The output of this component is coupled in series with the output from the high voltage limited power supply and of such phase that it adds to the HF guard voltage across the D.U.T., that is derived directly from the 4349B, thereby increasing the amplitude of this signal and the sensitivity of the xe2x80x9cpart presentxe2x80x9d detection circuit within the HP-4349B, without internal modification of this proprietary instrument in any way.
More, specifically, one aspect of this invention is a capability to increase the minimum sensitivity of the secondary or tertiary test of test instruments whose primary function is the testing of electronic components for their prime parameters, (resistors, capacitors or inductors), when that secondary or tertiary test is the detection of the presence of such a component within a test station and is performed by a superimposed high frequency signal passed through the device-under-test. By xe2x80x9csatisfactory connectionsxe2x80x9d is meant as being of such a low ohmic value as not to degrade the accuracy of the primary test or of any other function of the instrument.
Accordingly, the main object of this invention is a means of determining the presence of a DUT at the test station before the high resistance leakage test is commenced. Other objects of the invention include a device and method of using it to verify that the test circuit, for performing an electrical leakage test, is closed on a ceramic chip capacitor so that whatever value is recorded, it will be the value of the capacitor in the test station; a means of reducing the time taken in commencing and then terminating an electronic test for electrical leakage so as to increase the availability of the test machine for other capacitors; a means of increasing the throughput of ceramic chip capacitors in component handlers to reduce the operation time and, hence, the cost of producing quality ceramic chip capacitors; a means of lowering the cost of ceramic chip capacitors by increasing the efficiency of capacitor test machines; and, a means of reducing capital expenditures for test equipment by increasing the capability of ceramic capacitor testing machines to test for the presence of capacitors having a capacitance of 2.0 pF or less.
These and other objects of the invention may be determined by reading the description of the preferred embodiment along with the drawings attached hereto. The scope of protection sought by the inventor may be gleaned from a fair reading of the claims that conclude this specification.